The mystery that keeps on taking

So the mystery for me is what is taking away from my top end performance and from my gas mileage. I've had my 1999 Tacoma TRD for five years without ever having even the hint of a hiccup but I've got one now and, while I'd like to to do the work myself, my expertise on wrenching has been mostly on small British sports cars. I'm feeling like a duck and there's no water anywhere to be seen. So, here's what's up. I first noticed that I was suddenly going through a lot more fuel than usual. The computer also threw a code that could have pointed at a number of problems including leaving the gas cap loose. My local gas station has done that a couple of times to me so that might be all that was about. I cleared the code and it hasn't returned even after quite a few miles. But here's the real kicker, as I accelerate through the bottom three gears, I've got good acceleration right up to somewhere between 2,600 and 3,200 RPM and then it's like it just doesn't have any more "more" left in it. It just acts like I've reached the top of that gear and it can't go any faster, only, at those RPM's I'd think there would still be a lot left in that gear. The engine doesn't make any over reving sounds so I don't think it's a slipping clutch. I don't know if it's the injectors but if they were going to have a negative effect, I'd have thought that it would have shown up in the lower RPMs but again, I'm not that familiar with these critters. I'm not sure but it seems like I sense a bit of cutting out at the higher RPM's but it's so slight, it's hard to tell. The part that's really a mystery is that, whatever is causing the problems is only happening in the lower three gears, it doesn't seem to be at all effected in the top two gears. In those gears, it just keeps gathering speed and winding out to higher RPMs. Any thoughts or, are are there so many different possibilities that I should just take it to my Loacal Toyota dealer? And, as with all of these kinds of questions, thanks for any help anyone can provide and thanks even if you don't and you just took the time to read through this!

 

Maybe he should reset the ecu? Take off the battery for 20 min and let the truck relearn fueling if it is messing up. Anyone agree?

 

Start with the basics. Spark plugs, change em if they haven't been done in a while. Make sure you use the correct ones too - NGK 3967 - gap .043. This is assuming the 3.4L V6. Not sure what plugs the 4 cyls take.

Verify the spark plug routing is correct and that the wires aren't broken or having issues. There's a diagram of the proper routing floating around the forum if you search for it.

Make sure all the plugs are actually firing. If not, then you might have an issue with one or more of the coils on the passenger side.

Clean the throttle body and the MAF.

Replace the air filter and the fuel filter if they haven't been done recently.

O2 sensors going bad maybe?

Check for vacuum leaks. Lots of vacuum hoses under the hood. They can dry out and crack.

Basic stuff to start with.

 

Rough idle? Truck taking longer to turn on? Bad tank of gas? Is it winter atm? Do a tune up and see if it changes things if not time to diagnose, but dont throw money at parts, what did the code say?

 

Thanks all! To answer Tacoma 1997's questions, Idle is smooth and starts right up. Could be bad gas, but it runs fine with no cutting out in the lower revs so, that doesn't seem like it would be the culprit. Not sure what winter ATM is but it doesn't sound good. Is that anything like a truck's version of Seasonal affective disorder? unfortunately it threw the code almost a month ago and I didn't make a note of what it said. The gas cap did seem loose so I'm guessing it was the gas station not giving a good enough crank. Scary how old I feel when I realize how much has changed from the cars I'm used to working on. Injectors and Mass air flow sensor; Nope. O2 Sensor.... what's that?! Vacuum leaks, Nope again. All uncharted territory, but I now am familiar with all that stuff, have the appropriate unguents, notions, wrenches and threatening hammers and, keeping in mind your suggestions, I'm goin' in!

 

Has this problem just started or has been getting slowly worse ??

What kind of mileage drop are you talking ??

Has how you use the truck changed ??

Did the truck gain weight ??

Funny how an extra 1000 pounds behind the seat can effect fuel mileage.

Did you go to bigger then stock tires ??

Do you live where you have winter below freezing temps funny how that winter blend gas gets poor mileage and increased idle time melting the ice on the windows

When the MIL trips it is a good idea to take note of the code it can help point you in the right direction

 

I am still trying to figure out the bold statement... you mean there is a station that pumps gas for you? Outside of Japan, I haven't seen that in 20 years.

I vote MAF cleaning first to decrease chance the air/fuel ratio is not correct. Especially since that is easy and quick. Make sure to disconnect battery as suggested above. Otherwise it'll take longer to re-adjust if that is in fact part of the problem.

 

Hi Will,
Little British cars huh?
We have a 76 MGB.
At first I was thinking a coil is going bad.
But if top two gears are fine I ruled that out.
What about the air intake pathway?
Could there be an obstruction of some kind that's intermittent? Critters nest or rag?
Probably not likely.....but I'd check the airbox and fenderwell where the air comes in.

Try running without the air filter and see if speed in mid range goes back to normal.

 

no to hijack but there is one station near my grandparents that is usually cheaper than everyone else and is still full-service

 

to start with
how many miles on the engine?
are the tires oversized?
2.7 or 3.4?
does she use any oil?

 

radical gas mystery here too!

since i got some new tires (bigger than the stock ones) a little over a year ago i can 'barely' get to 20 mpg on a tank, does not matter the octane grade either
earlier this week I was in Colorado, much higher altitude than at home, fast paced traffic 80-90mph, climbing up the mountain in 5th gear for 200 miles
I used Shell 91 octane while i was there, like i occasionally use at home 500 miles away,

truck had 350 miles on the tripmeter and the low fuel light warning had not come on yet, meaning i could get more than 400 miles on the tank and over 23 MPGs

what is this mystery of losing over 50 miles per tank at home

1. was it the higher altitude and thinner air helping my engine?
2. is Shell making more 'pure' gasoline for the Denver area (but i think it had 10% ethanol even in the 91 octane) than they supply me with locally that supposedly has no ethanol added to the 91?

i am very confused by this experience
i know the faster i go over 55 / 65 the poorer the MPGs get but i was definitely not puttering my way around on I-25

 

As Wyoming said, start with the basics.

Plugs, wires, throttle body clean, MAF clean, IAC valve clean, etc...

It's not likely to be things like your coils or a bad O2 censor, as those would typically throw error codes.

On a side note, do the 1st gen tacomas even have the "gas cap" error code? I didn't think they had that advanced of an emissions system to check the fuel system for leaks. I know my '14 Corolla and '17 4Runner do, but my 2004 doesn't have that. You can hear the pumps running from time to time.

 

The reason you're not getting better mpg on 91 is because higher octane doesn't do anything on a taco. But going to higher tires is definitely an issue with performancev and mpg.

 

if that is in regards to my using 91 octane is there any way to explain the recent results I had from being in Colorado, Kansas and Nebraska?
a clear 3 more MPGs per full tank was made in eastern Colorado
i did not change tires while crossing the state borders lol

the lowest octane CO had i think was 85
and i recall the last time i put that low grade stuff in my engine would ping or make weird noises at lowish RPMs but it has been awhile since i used that grade, i usually do the 87 with ethanol because of the 90c price difference here compared to the 91

 

I don't argue octane or abortion... But octane is pure physics and thermodynamics. If your engine NEEDS high octane to run correctly it has issues that are only being bandaided.

I lived in Cheyenne most of my life, 85 was the norm for that elevation. I'm Sea level now, it's 87.

 

Not to hijack the thread, but this sounds like a matter of doing all highway miles and the efficiency of maintaining a constant speed instead of driving around town (at home).

 

you would think! but NO not the case i am fairly certain
traveled I-70 straight through Kansas to Salina from Colorado (250+ miles) with fillup at Burlington and got about 19.33 at variable speeds to make sure i got to my destination at the agreed time
the mystery that keeps on taking..

 

I’d say clean the MAF sensor for starters. I recovered a lot of power when I cleaned mine. Also check to see if you have any pending codes. Sometimes I find a pending code when my check engine light is not on. Good luck!

 

Wait, I though 91 octane added like 35 HP???

 

the different states might have an ethanol content???